Re: Regarding Piping Stress Analysis

On Feb 23, 2005, at 10:52 PM, sathish babu wrote:

> When a Forces and moments to be checked ?
The short answer is 'always.' Piping system stress must be kept within code limits for all loadings and all service conditions. Verification that you have not exceeded code limits is your responsibility as an engineer. You need not always do a complete stress analysis of every last detail, but you must be able to satisfy yourself, using judgement based on experience, that all the details will meet the code. CAESAR isn't enough by itself, because it provides results based on idealizations with limited validity.

In your case you idealized the valve as rigid, which it is with respect to the piping network, but the valve body isn't absolutely rigid. Loads applied to the valve cause deformations and stresses which must also meet code requirements. Normally design of the valve is the responsibility of the valve manufacturer, but the manufacturer can't possibly anticipate every possible service condition the valve might see, and it sounds like he asked you for an assessment of the valve under your particular service. Did I guess right? If so, do you have reason to believe that your service is such that the valve body might be over-stressed?

Again, guessing, I suspect that what you call 'allowable moments' are in fact calculated moments from CAESAR which turn out to be allowable because they don't exceed the maximums allowed by the code for your loading. In that case they'd be allowable moments for the pipe, but not necessarily allowable moments for the valve. To know that you'd need to do a stress analysis of the valve body. Presumably that's why the manufacturer 'produced the drawing,' by which I presume that he produced a drawing of the valve body.

In fact you should probably be asking the manufacturer for guidance, and be prepared to help out the some extent, at least with an estimate of valve body stress caused by your particular combination of forces and moments. Doing a complete job of it may be beyond your scope of work, because it could be expensive, but you may have to do something if you want to use that particular valve. This is your piping system, not the valve manufacturer's, so you have the major responsibility and interest for insuring that the valve provides satisfactory service. The valve manufacturer's customer support should be willing to help to a degree--if he isn't interested in helping, maybe it's time to tell him you're shopping elsewhere for valves.

(As an aside, here's one of those engineering issues which demands accurate written communication, whether in English or Hindi or whatever. You can't do it with a picture. I've guessed at a lot of the issues Satish is facing, and I may have gotten it totally wrong. Satish's English is good, and what I said is correct, but I might not have answered the question or my answer might be the answer to the question as posed, but the question posed might not be the question he meant to ask. Or it might be the question he meant to ask but not the question he should have asked so that I might infer the intended question and thus give him the answer he needed, which of course might not be the answer he expected. That's perfectly clear, isn't it? ;->)

Christopher Wright P.E. |"They couldn't hit an elephant at chrisw@skypoint.com | this distance" (last words of Gen.

.......................................| John Sedgwick, Spotsylvania